1. Field of the Invention
The present invention relates to an oxide high-temperature superconductor, and more particularly, to a Bi-Sr-Ca-Cu-O system superconducting thin film and a method of forming the same.
2. Description of the Related Art
Attempts have been made to adapt their superconductors to devices or wirings thereby using superconduction properties (e.g., zero electric-conductivity). However, since a critical temperature (Tc) of the superconductors is very low, they can be used only in liquid helium (He). Recently developed oxide superconductors have a relatively high Tc, so that they can be used in liquid nitrogen (N). Furthermore, an oxide high-temperature superconducting film can be formed by depositing the oxide on a substrate under an atomic layer control, and such a film formation has been attempted and studied (cf., Tomoji KAWAI: "Design and Synthesis of High Tc Superconducting Superlattice by Successive Deposition Method", Bulletin of the Japan Institute of Metals, Vol. 29, NO. 9 (1990), pp. 733-739, and J. N. Eckstein et al., "Epitaxial growth of high-temperature superconducting thin films", J. Vac. Sci. Technol. B7 (2), 1989, pp. 319-323).
In contrast to metal superconductors, the oxide high-temperature superconductors have a problem, in that its coherent length (.xi.c) in the C-axis direction is remarkably short. For example, it is presumed that the respective coherent lengths of a Y-Ba-C-O system oxide and a Bi-Sr-Ca-Cu-O system oxide are about 0.3 nm and 0.1-0.2 nm, and thus these coherent lengths are very small compared with coherent lengths of the metal superconductors (e.g., 100 nm of Pb and 40 nm of Nb). Therefore, when a magnetic field is applied to the oxide superconductor in the C-axis direction, the critical temperature Tc is remarkable lowered.
In Bi system oxide superconducting films (e.g., Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.x), the partitioning Perovskite structure layer of the BiO layer serves as an electron-shielding (blocking) layer, and electrons of CuO.sub.2 layers which clearly afford the superconductivity function can scarcely pass through the BiO layer, thereby shortening the coherent length (.xi.c) of the Bi-Sr-Ca-Cu-O system oxide. If the number (m) of CuO.sub.2 layers in a half-unit cell of the Perovskite structure layer is increased, the coherent length can elongate. However, when the number (m) of the CuO.sub.2 layers is 4 or more, the critical temperature falls, and at m=5 it becomes 10.degree. K. or less (namely, such an oxide superconducting film necessitates helium cooling).